The present invention relates to cables and, more particularly, to a hybrid cable for concurrently carrying optical signals and electrical signals.
Seismic surveying is commonly employed to evaluate the feasibility of oil exploration at a candidate site. One method of such surveying includes the manual deployment of an array of geophones connected at spaced locations along a cable network, to measure the reflections of the acoustic waves to an event such as detonation of an explosive charge. The geophysical cable used has a large count of twisted pairs of conductors (e.g., 126), one pair for each geophone, for carrying signals from the geophones to central processing equipment. The use of this system is limited by the need for manhandling the cable because the cable is heavy and bulky due to the large number of conductors, and because each conductor pair is dedicated to a respective geophone.
An improvement in this surveying method involves the use of a number of serially connected cable segments each having a limited number of twisted pairs (seven, for example). At the end of each cable segment, the analog signals from the respective twisted pairs are converted by an analog to digital converter and combined by multiplexer means into a pulse train carried by other conductors in the cable. Thus the twisted pairs in the next cable segment can be connected to other geophones. The cables used are much smaller and lighter, and the number of geophones is limited not by the cable, but only by the capabilities of the digital electronics used in the system. The cable segments are placed in series along the ground with the entire array of interconnected cable segments called a "spread". As the surveying advances in a predetermined direction, the spread must be moved along the ground. Often, particularly where the terrain is rugged, this movement is accomplished by unskilled hand labor. Thus it is desired that the cable segments be as light as possible. A recording truck is used to carry the processing equipment and it may be connected into the spread at random at any convenient location. It is of course unavoidable that the truck will occasionally run over the cable and it must have sufficient strength to withstand the crushing forces.
Attempts to develop a suitable cable for use in the improved method have not been entirely satisfactory. Cables using all metallic conductors and hybrid cables having both metallic conductors and optical conductors have been suggested. Optical conductors are preferred for carrying digital signals. Since the optical conductors have a greater bandwidth, a higher data transmission rate can be achieved. The great temperature range in which seismic surveying is conducted, as in hot deserts (where the temperature can reach 120 degrees Fahrenheit) and cold arctic (where the temperature can fall to minus 70 degrees Fahrenheit) and the unavoidably rough handling of the cable and equipment place especially stringent requirements on the cable. It was found necessary in first using the improved method to employ a cable having triple redundant optical conductors, because incidents of optical conductor breakage were unacceptably high. Of course, the use of a triple redundant system requires complex electronics, such as a majority vote circuit, to accept signals on any two out of three optical conductors as representing the valid signal. Reference may be made to U.S. Pat. No. 4,117,448 for a description of such prior geophysical prospecting arrays and systems and their manner of operation.